Processing of leather essentially involves pre-tanning, tanning and post-tanning operations followed by finishing. Pre-tanning involves a series of operations like soaking, liming, deliming, bating and pickling that essentially condition the hides and skins for subsequent tanning.
Leathers require to be dyed in different shades depending on the requirements of the consumer products to be made therewith. Dyeing of leather has conventionally been a post-tanning wet operation, which is usually carried out using different conventional dyestuff like acid dye, direct dye, metal complex dye etc. The emergence of the environmental concern in respect of the hazardous components associated with these dyestuffs has prompted the researchers to explore possibilities of evolving eco-benign options for colouring of leathers.
It has been an established fact that different tanstuff used for tanning impart certain colour onto the tanned leather. Vegetable tanning materials, which have traditionally been of use for leather tanning, produce brown leathers. It has been possible to produce vegetable tanned leathers of different shades by using metallic salt as striker. For example, vegetable tanning materials, in combination with iron salt, produce black leathers as studied by Raghava Rao et al (Journal of Society of Leather Technologists and Chemists, 86, 106-111, 2002).
While there has been no report on producing pink leather using vegetable tanning method along with any metal striker, the major limitation associated with the vegetable tanning materials is that they lead to excessive loading, which reduces the versatility of the resulting leather to make different end products. Moreover, vegetable tannins are also known to be hard biodegradable materials.
With the advent of the mineral tanning agents, chromium has emerged as the most popular tanning agent, which results in blue coloured leather, popularly known as wet blue. In this process, pickled pelts (hides/skins) are tanned using basic chromium sulphate. While chromium(III) salts find extensive usage in leather processing, the environmental concerns due to chromium pollution has led to the search for alternative tanning agents.
Among other mineral tanning agents, Selverangam et al (Leather Science, 11, 383, 431, 1964) studied the potential of aluminium for tanning; Covington (Journal of American Leather Chemists Association 82, 1, 1987) studied the tanning potential of titanium; Gaidau et al (Journal of Society of Leather Technologists and Chemists 82, 143, 1998) reported the tanning potency of iron and Sreeram et al (Journal of American Leather Chemists Association, 95, 324, 2000) explored the possibility of zirconium as a tanning agent. However, each one has inherent disadvantages associated with them as highlighted by Madhan et al (Journal of American Leather Chemists Association, 97, 189, 2002). Though aluminium and titanium tanning produces white leathers they have poor hydrothermal stability. Iron tanned leathers undergo darkening of colour during ageing and also have poor strength characteristics.
Zirconium-tanned leather is usually fuller and firmer than those produced by chrome tanning. Sreeram et al patented a process for the preparation of novel organo-metallic polymeric matrix based on zirconium (Indian Patent Application No. 3077/DEL/98). The leather so produced is of a pleasing white color, has good light fastness, and is superior to alum-tanned leather and has a higher (above 90° C.) shrinkage temperature. Zirconium salts tend to be very astringent, and normally produce tight, firm leather; causes rapid tannage of the grain and produce a fine, short nap on suede leathers. Through the use of masking salts, such as acetates, the astringency can be reduced, resulting in soft, smooth-grained leather as studied by Madhan et al (Journal of American Leather Chemists Association 98, 107-114, 2003). The major limitation associated with the zirconium tanning is that the zirconium salts precipitate as basic salts at a pH higher than 2.0. Another limitation associated with this tannage is that zirconium tanning is known to produce leathers with drawn grain. However, reports are not prevalent on the adverse effects or toxicity of zirconium salts. Hence, tanning using zirconium salts would be an eco-friendly option.
Studies on tanning properties of Tetrakis hydroxymethyl phosphonium salt (THP) were carried out by Das Gupta (Journal of Society of Leather Technologists and Chemists, 86,188, 2002) as an alternative-tanning agent to the traditional chrome tanning system. Burrow et al studied the tanning potential of THP with chromium (U.S. Pat. No. 6,685,747). The effects of THPS on the shrinkage temperature of lambskins and their combinations with aluminium salts were studied. Benefits of THPS include low toxicity, low treatment levels, rapid breakdown in the environment, and no bioaccumulation. When substituted for more toxic biocides, THPS biocide provides reduced risk to both human health and environment.
No prior art is available on the production of pink coloured leather at the tanning stage without using any dye or pigment.